The invention relates to amplifier and coupling circuitry for electret microphones.
Electret microphones are commonly used in headsets for telephony, computer telephony and speech recognition applications. Such electrets have virtually displaced the larger, heavier dynamic microphones. Dynamic mics generate a very small signal voltage when excited by voice. The electret mic provides higher output by virtue of active circuitry (most commonly a JFET), but requires a DC bias voltage source that may be part of the telephone or computer.
Low cost electret mics may suffice for telephony because of the limited telephone link bandwidth, the wide dynamic range of the human ear and the ability of the brain to compensate for noise and distortion. Expensive, high performance electrets are preferred in voice enabled applications with personal computers such as voice command and speech recognition because they contribute to greater accuracy. The microphone or headset is connected to a sound card and in almost every instance today, the PC provides DC bias at the mic receptacle, which is the familiar 3.5 mm stereo jack also found on other consumer electronic products. It is desirable to provide the greater recognition accuracy of high performance electrets using low cost mic elements by augmenting with circuitry and electroacoustic techniques.
Methods to enhance the performance of ordinary, low cost electrets using batteries and circuitry are disclosed in co-pending U.S. patent application Ser. No. 08/968,307 and U.S. patent application Ser. No. 08/967,930 of common assignee. Battery based consumer products, however, have drawbacks, more so when the batteries are not included in the package. Although as stated, the PC sound card provides DC mic bias via the microphone input connector, there is no standard with respect to voltage level or pinout at this stereo jack. Although there is only one connector for one microphone, xe2x80x9c3.5 mm stereoxe2x80x9d in this instance describes a jack to accept a plug with three terminals: tip, ring and sleeve. Sleeve is always the common or ground terminal, but tip and ring may be wired in one of several ways for DC bias and signal. In addition, the low voltage bias source may be severely current limited by high resistance internal to the sound card, making the design of external, active circuits to enhance mic performance very difficult. In fact, even without additional circuitry, mic operation may be compromised by the limited current from the computer.
In one embodiment of the invention there is provided a circuit for connecting an electret microphone to audio inputs. The circuit includes a coupling circuit for accepting DC bias voltage from the audio inputs and providing audio signal to the inputs. Frequency shaping circuitry is provided for reducing unwanted microphone sensitivity at selected frequencies. In one aspect of the invention a battery connector is provided where at least one battery may be optionally connected.
In each embodiment of the invention, the circuit output impedance can be made lower to minimize electret mic attenuation caused by an output node preferential to dynamic microphones. Each aforementioned and one further, simpler embodiment will operate without regard to the wiring configuration of the audio output node, with no additional hardware and requiring no special installation steps by the user.